Reversible molecular motion of a bis-calix[5]arene host driven by a photoresponsive guest

Article abstract of DOI:10.1002/asia.201100738

Control by light: Reversible switching between closed and open conformations of a calix[5]arene-based receptor was conveniently controlled, at the supramolecular level, exclusively through light inputs by exploiting the photoisomerization of a tailor-made azobenzene guest.

Full text of DOI:10.1002/asia.201100738

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DOI: 10.1002/asia.201100738
Reversible Molecular Motion of a Bis-calix[5]arene Host Driven by a
Photoresponsive Guest
Giovanna Brancatelli,[a] Calogero Capici,[b] Giuseppe Gattuso,[b] Silvano Geremia,[a]
Anna Notti,[b] Sebastiano Pappalardo,*[c] Melchiorre F. Parisi,*[b] Salvatore Sortino,*[d] and
Elisa Vittorino[d]
Dedicated to Prof. Mario Gattuso on the occasion of his retirement
Modulation and control of the molecular features of a
given receptor/substrate, and hence their supramolecular
properties, by means of external, nondestructive, stimuli is
still a topic of current and increasing interest, not only from
a fundamental perspective but also from that of future nano-
devices.[1] Light is the most appealing on/off trigger. The
combination of its ready availability and easy manipulation,
together with the fast rate of photochemical reactions,
makes photoregulated systems particularly suited for multi-
faceted applications in different fields, which range from
molecular optoelectronics[2] and energy conversion,[3] to the
environment[4] and biomedicine.[5]
Control over host–guest complexation events is one of the
fundamental areas in which photoresponsive centers play a
key role. In this respect, the reversible trans–cis photoisome-
rization of azobenzene moieties has proved to be one of the
most convenient tools for inducing conformational modifica-
tions that may influence functional properties at the nano-
scale level.[6] Incorporation of an azobenzene moiety either
into a receptor framework[7] or into a substrate[8] has provid-
ed orthogonal triggers for the selective capture of target
species and/or controlled release of bound substrates. Very
recently, the use of azobenzene cores has allowed control
over the reversible assembly/disassembly of capsular com-
plexes.[9]
As part of our ongoing interest in calix[5]arenes as selec-
tive receptors for linear a,w-alkanediyldiammonium
guests,[10,11] we have demonstrated that receptor 1 (Figure 1),
composed of two calix[5]arene units covalently linked to
each other through their upper rims, forms either capsular
or noncapsular 1:1 complexes according to the molecular
length of the alkanediyldiammonium guests present in solu-
tion.[12]
Bis-calix[5]arene 1 is a heterotetratopic receptor capable
of simultaneously and synergically binding, with remarkable
efficiency, linear alkanediyldiammonium dications along
with their counterions by means of the ureido moieties lo-
cated in the phenylene bridging unit. Depending on the
length of the alkyl chain of the diammoniun dication, recep-
tor 1 adopts in solution (CDCl3/CD3OD, 4:1) either an open
“S-shaped” conformation, in which the two p-rich cavities
arrange themselves in an antiperiplanar fashion with respect
to the phenylene spacer, or an opposite closed “C-shaped”
synperiplanar conformation, to allow either one or both ter-
minal ammonium moieties of the substrate to nest inside the
[a] Dr. G. Brancatelli, Prof. S. Geremia
Centro di Eccellenza in Biocristallografia
Dipartimento di Scienze Chimiche e Farmaceutiche
Universitꢁ di Trieste
bis-calixarene cavityACHTNUTGRNEUNG
(ies) (Figure 1a).[12]
Via L. Giorgieri 1, 34127 Trieste (Italy)
Intrigued by this behavior and motivated by the above
considerations, herein, we have explored the possibility of
reversibly controlling the openQclosed conformational in-
terconversion of host 1 without changing the nature of the
guest,[13] using instead photons alone as external triggers.
Based on an inspection of the solid-state structure of 1 (see
below),[14] diammonium salt 2·2HCl (incorporating a photo-
chromic azobenzene moiety in its skeleton) was designed
and synthesized as a potential guest. The shape/length of
this photoactive guest can be modulated by the trans–cis iso-
merization of its azo group.[15] When it adopts the trans con-
figuration, the guest has the correct shape (antiparallel di-
topic guest with a nitrogen-to-nitrogen distance of 16.6 ꢀ;
Spartan’10[16]) to encourage the formation of the closed “C-
shaped” capsular complex through a positive self-assembly
cooperativity.[17] On the other hand, the more compact
[b] Dr. C. Capici, Dr. G. Gattuso, Dr. A. Notti, Prof. M. F. Parisi
Dipartimento di Chimica Organica e Biologica
Universitꢁ di Messina
Viale F. Stagno d’Alcontres 31, 98166 Messina (Italy)
E-mail: mparisi@unime.it
[c] Prof. S. Pappalardo
Dipartimento di Scienze Chimiche
Universitꢁ di Catania,
Viale A. Doria 6, 95125 Catania (Italy)
E-mail: spappalardo@unict.it
[d] Prof. S. Sortino, Dr. E. Vittorino
Laboratory of Photochemistry
Department of Drug Sciences
University of Catania
Viale A. Doria 6, 95125 Catania (Italy)
E-mail: ssortino@unict.it
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100738.
50
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Chem. Asian J. 2012, 7, 50 – 54

Single-crystal[18] X-ray analy-
sis of bis-calix[5]arene
1
showed the existence of two
different open conformational
isomers in the unit cell: one
molecule of the antiperiplanar
“S-shaped” isomer, 1a, lying on
a
crystallographic inversion
center (Figure 2a), and two
molecules of a clinal “gauche-
shaped” conformer, 1b (Fig-
ure 2b; see also the Supporting
Information, Figures S1 and
S2). In the “S-shaped” isomer,
the five linking methylene
carbon atoms of each calix[5]ar-
ene subunit are coplanar within
0.436 ꢀ, and the distance be-
tween the two mean planes is
15.98 ꢀ. Each calix[5]arene
moiety hosts a dimethyl sulfox-
ide (DMSO) molecule posi-
tioned within the cavity with a
methyl group pointing towards
the center of the macrocycle.
The DMSO molecule simulta-
neously interacts both with a
urea NH group through
a
NH···O hydrogen bond [with an
N···O distance of 2.861 ꢀ] (Fig-
ure 2a) and with three phenyl
rings
on
the
macrocycle
À
through several C H···p inter-
actions, which are listed in
Table S2 in the Supporting In-
formation. Conversely, in 1b
only one of the two calix[5]ar-
ene cavities is filled with
a
DMSO molecule, whereas the
empty one shows a very distort-
ed cone conformation. The two
calix[5]arene subunits are posi-
tioned in
a
clinal gauche-
shaped conformation, with
a
torsion angle Cg-C(6a)-C(6d)-
Cgꢃ of À808 with respect to the
phenylene bridge (in which Cg
and Cgꢃ are the gravity centers
of the calix[5]arene macrocy-
cles; Figure S2 in the Support-
ing Information). In the crystal
lattice the two isomers are held
Figure 1. Conformational arrangements of 1 in the presence of ÀCl+H3N-(CH2)n-NH3+ClÀ (a) and 2·2HCl
(b) in CDCl3/CD3OD, 4:1 and 5:1, respectively.
together by
strong, highly directional (and
thus selective) intermolecular hydrogen-bonding interac-
tions between the two 1,4-bis(ureido)phenylene spacers. The
NH donor groups and the carbonyl acceptor oxygen atoms
a
network of
isomer with the cis configuration of the azobenzene moiety
will not fit within the “C-shaped” ditopic host and is expect-
ed to favor the open conformation of 1.
AHCTUNGTRENNUNG
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51

S. Pappalardo, M. F. Parisi, S. Sortino et al.
COMMUNICATION
1
unequivocally demonstrated by a combination of H NMR
spectroscopy experiments and UV/Vis absorption spectra re-
corded under different experimental conditions, as shown in
Figure 3.
Figure 2. Molecular structures of: a) the “S-shaped” Ci-symmetric bis-cal-
ix[5]arene molecule (1a) and b) the “gauche-shaped” bis-calix[5]arene
molecule (1b) present in the crystals of 1 (the appending isohexyl groups,
which adopt an extended conformation, have been omitted for clarity).
Figure 3. A) Sections of the 1H NMR spectra (500 MHz, CDCl3/CD3OD,
5:1, 298 K) of: a) [1]=1 mm; b) [1]=[2·2HCl]=1 mm; c) the sample from
trace b after 30 min of irradiation at lex =330 nm; d) the sample from
trace c after 4 min of irradiation at lex =440 nm. The asterisk indicates re-
sidual solvent peak, (!) and (!) refer to trans-[2·2H]2+ꢀ1 and cis-
[2·2H]2+ꢀ1, respectively; for peak assignments, see Figure S7 in the Sup-
give rise in one case to a bifurcated hydrogen bond, typical
of urea, and in the other one to a more extended hydrogen-
bonding pattern, involving also a bridging DMSO guest mol-
ecule (Figure S3 in the Supporting Information).
porting Information. B) Absorption spectra of
a solution of [1]=
Herein, we demonstrate that photoisomerization of
2·2HCl can successfully be employed to operate a transfer
of conformational information at the supramolecular level,
thus allowing repeated closedQopen motions by means of
alternate light inputs of different energy (Figure 1b).
[2·2HCl]=0.3 mm recorded upon intervals of irradiation at lex =330 nm
(left) and subsequent irradiation at lex =440 nm (right). The spectra were
recorded at room temperature using a quartz cell with an optical path
length of 1 mm. The arrows indicate the spectral evolution with the irra-
diation time.
Firstly, we tested the photobehavior of 2·2HCl in a
CDCl3/CD3OD solution. Irradiation with 330 nm light re-
sulted in bleaching of the main absorption band matched by
the formation of a new absorption band at 435 nm (Fig-
ure S5A in the Supporting Information). A photostationary
state (approximately 85% conversion of the trans isomer)
was reached after 15 minutes of irradiation.[19] Further irra-
diation of this solution with 440 nm light led to nearly com-
plete recovery (up to 95% of the initial intensity) of the
original band at 330 nm (Figure S5B in the Supporting In-
formation). These results are in full agreement with a rever-
sible transQcis photoisomerization of the azobenzene
moiety, thereby suggesting that the functionalization of both
phenyl rings (with ethyl ammonium groups) does not affect
its photochromic properties. The cyclic nature of the light-
induced conformational switching depicted in Figure 1b was
Upon addition of an equimolar amount of trans-2·2HCl,
the host readily rearranges from an open to a closed “C-
shaped” conformation to simultaneously accommodate
inside its cavities both ammonium moieties of the guest
(Figure 3A, traces a,b).[20] Bis-endo-cavity inclusion of
[2·2H]2+ within 1 (trans-[2·2H]2+ꢀ1) is consistent with the
presence, in the high-field region of the 1H NMR spectra
(d=À0.55 to À0.35 ppm), of two multiplets belonging to the
a- and b-methylene groups (8H in total) of the diammonium
guest (experiencing the shielding of the two calixarene aro-
matic units),[21] and with a doublet at d=5.50 ppm assigned
to the included 3,5,3’,5’ azobenzene hydrogen atoms.[22] Inte-
gration of peaks pertinent to the host and the included guest
reveals that complexation is nearly quantitative. Irradiation
of this complex with 330 nm light[23] induces the trans-to-cis
52
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Chem. Asian J. 2012, 7, 50 – 54

Reversible Molecular Motion of a Bis-calix[5]arene Host
isomerization of the azobenzene guest, as confirmed by the
typical evolution of the UV/Vis spectra over the whole time
course (Figure 3B, left).
A photostationary state was
reached after 26 minutes of irradiation. This process causes
the disruption of the capsular complex trans-[2·2H]2+ꢀ1,
with the consequent formation of a monocavity complex cis-
[2·2H]2+ꢀ1, in which the receptor most likely adopts again
an open conformation as a result of steric hindrance be-
tween the isomerized guest and the empty cavity of the
host.[24,25] The 1H NMR spectrum recorded immediately
after reaching the photochemical equilibrium shows in fact
the presence of new sets of resonances, which belong to the
desymmetrized cis-[2·2H]2+ guest (i.e., with one of the am-
monium moieties inside and the other outside of a calixar-
ene subunit of 1) and receptor, that is, with one of the two
cavities “empty” and the other “filled” (Figure 3A, trace c).
Integration of the peaks accounting for the two complexes
reveals that, in the photostationary state, approximately
55% of the initial capsular “C-shaped” complex has been
converted into an open monocavity complex.
The changes observed in both the kinetics (see also Fig-
ure S6 in the Supporting Information) and the equilibrium
position (trans/cis ratio) of the photoisomerization process
for the included trans-[2·2H]2+ dication, if compared with
the free salt, are not uncommon for an azobenzene deriva-
tive confined in a restricted microenvironment and/or con-
strained media. Indeed, both effects can be the result of the
steric hindrance of the receptor cavity to the whole photo-
chemical mechanism of photoisomerization.[26] The reversi-
bility of the host conformational switching was confirmed by
irradiation experiments carried out with lex =440 nm. The
nearly complete recovery (up to 95% of the initial value) of
the original band at 330 nm, observed after four minutes of
irradiation (Figure 3B, right), confirms that the configura-
tion of the guest has reverted from cis to trans, thus restor-
ing the capsular complex trans-[2·2H]2+ꢀ1 (Figure 3A,
trace d).
The adaptable nature of the photoisomerization cycle, il-
lustrated in Figure 1b, is demonstrated by a parallel experi-
ment in which the irradiation/addition sequence was invert-
ed. Accordingly, irradiation of a CD3OD solution of trans-
2·2HCl with 330 nm light led to an approximately 85% cis
isomerization of the dication (Figure 4, traces a,b and Fig-
ure S5A in the Supporting Information). Prompt addition of
a CDCl3 solution of 1 resulted in 77% of cis-[2·2H]2+ꢀ1
and 23% of trans-[2·2H]2+ꢀ1 (Figure 4, trace c).[27] When
this mixture was irradiated with 440 nm light, the cis dicat-
ion fully reverted to the trans isomer producing, almost
quantitatively, the trans-[2·2H]2+ꢀ1 capsular complex
(Figure 4, trace d), which in turn, upon further irradiation
for 26 minutes at lex =330 nm regenerated the open mono-
cavity complex (Figure 4, trace e) observed in the previous
complexation/irradiation experiment.
Figure 4. Sections of the 1H NMR spectra (500 MHz, 298 K) of: a) trans-
[2·2HCl]=1 mm in CD3OD; b) the sample from trace a after 15 min of ir-
radiation at lex =330 nm; c) the sample from trace b after addition of 1
(trans-[2·2HCl]=[1]=1 mm in CDCl3/CD3OD, 5:1); d) the sample from
trace c after 4 min of irradiation at lex =440 nm; e) sample from trace d
after further irradiation at lex =330 nm (26 min). The asterisk indicates
residual solvent peak; (!) and (!) refer to trans-[2·2H]2+ꢀ1 and cis-
[2·2H]2+ꢀ1, respectively; for peak assignments, see Figure S8 in the Sup-
porting Information.
energy absorbed by a single, tailor-made photoresponsive
guest. The approach presented herein may, in principle, be
useful for reversibly controlling the assembly/disassembly[11]
of supramolecular nanostructures made of homoditopic
complementary host and guest molecules exclusively by ex-
ternal light inputs. Studies in this direction are currently in
progress in our laboratories and the results will be reported
in due course.
Experimental Section
CCDC 836621 contains the supplementary crystallographic data for com-
pound 1. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found in the
online version.
Acknowledgements
MIUR (PRIN-2008A9C4HZ and PRIN-2009A5Y3N9 projects) is grate-
fully acknowledged for financial support of this research.
Keywords: azo compounds
·
calixarenes
·
host–guest
systems · photochemistry · X-ray analysis
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Chem. Asian J. 2012, 7, 50 – 54
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[14] Compound 2·2HCl was prepared in two steps from N-acetyl-p-nitro-
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Klunzinger, A. M. Lee, M. S. Lee, W. Liang, I. Lotan, N. Nair, B.
Peters, E. I. Proynov, P. A. Pieniazek, Y. M. Rhee, J. Ritchie, E.
Received: September 1, 2011
Published online: October 31, 2011
54
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Chem. Asian J. 2012, 7, 50 – 54